Natural gas serves as the energy source for much of the currently generated electricity. To this end, the gas undergoes combustion in a gas turbine which powers an electrical generator. However, the products of combustion leave the gas turbine as an exhaust gas quite high in temperature. In other words, the exhaust gas represents an energy source itself. This energy is captured in a heat recovery steam generator (“HRSG”) that produces superheated steam that powers another electrical generator.
An HRSG in its most basic form includes a casing having an inlet and an outlet and a succession of heat exchangers—namely a superheater, an evaporator, and a feedwater heater arranged in that order within the casing between the inlet and outlet. Subcooled water enters the HRSG at the feedwater heater which elevates its temperature. The warmer water from the feedwater heater flows into the evaporator where it is converted into saturated steam. That steam flows on to the superheater which converts it into superheated steam, and, of course, the superheated steam enters the steam turbine.
Each heat exchanger includes coils, having tubes, usually oriented vertically and arranged in rows across essentially the entire width of the casing. To be sure, the coils must be somewhat narrower than the spacing between the liners that form the inside surfaces of the sidewalls of the casing, this in order to facilitate installation of the coils in the casing. In this regard, the coils are typically lowered through the roof of the casing to the floor of the casing, whereupon they are secured in the casing. Owing to the side clearances required for the installation, gaps exist between the endmost tubes of the coils and the liners along the sidewalls of the casing. Unless these gaps are obstructed, some of the exhaust gas flowing through the casing will simply bypass the coils, or in other words, flow past the sides of the coils instead of through the coils. The exhaust gas that bypasses does not transfer its energy to the water in the coils, whatever phase that water may be.
Typically, once a coil is lowered into its position within a casing, the gaps to the sides of it are closed with baffles which are formed from steel angle iron. Traditionally, sidewall gas baffles are constructed of ¼ in.×2 in.×2 in. steel angle iron. They are welded to the liner plates after the coil modules are installed through the roof of the casing. One flange of each baffle lies against the sidewall liner at the gap, while the other flange projects toward the coil and closes the gap.
Hence, such sidewall gas baffles in an HRSG extend from top to bottom on the left and right side of the heat transfer coils where they butt up against the inside liner. Their purpose is to prevent exhaust gases from taking a shortcut between the heat transfer coil and the liner wall. Thus, eliminating the exhaust gas flow shortcut of bypassing the coil increases the heat exchange performance of the HRSG.
But installing a traditional angle iron baffle is a time-consuming procedure. A contractor normally needs to erect scaffolding between the coil modules to have a platform in which to weld these angle baffles. Hence, it requires workers to enter the casing, erect scaffolding, and weld the angles to the liners, with much of the work being conducted from the scaffolding.
These baffles are installed separately in the field because the tolerances in the construction of the HRSG are too tight to prefabricate these angle baffles either on the coil or on the liner, and ensure that the coils can be lowered in from the roof without inflicting damage.
An advantage of this inventive system and method is that the sidewall gas baffle can be installed onto the coil module in the shop. A mount is positioned on the coils. The baffle is supported by the mount. The baffle is retained in a first position relative to the mount and coil by a retainer. The retainer can be released so that the weight of the baffle will cause the baffle to slide down and outward to a second position, bridging the construction tolerance gap to block bypass of the exhaust gap in an HRSG. In a preferred embodiment, during construction, and installation of the coil modules, the baffle is held in the first position by ties. These ties will melt at operating temperatures of the HRSG. When the baffles are so held, such as by ties, in the first position, they fit within the profile of the coil module, allowing the coil to be dropped in through the casing, as is normally done.
The foregoing and other features and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.